1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) device and more particularly, to an active-matrix addressing LCD device of the lateral electric field type, such as the In-Plane Switching (IPS) type. The present invention is applicable to monitors designed for computers using the lateral electric field type LCD device, LCD televisions, portable telephone terminals, Global Positioning System (GPS) terminals, car navigation systems, video game machines, Automatic Teller Machine (ATM) terminals located at banks or convenience stores, medical diagnosis apparatuses, and so on.
2. Description of the Related Art
Generally, the LCD device has the features such as low profile, reduced weight, and low power consumption. In particular, the active-matrix addressing LCD device that drives the respective pixels arranged vertically and horizontally in a matrix array by the active elements has ever been recognized as a high image quality flat-panel display device. Especially, the active-matrix addressing LCD device using thin-film transistors (TFTs) as the active elements for switching the respective pixels has been extensively diffused.
Most of active-matrix addressing LCD devices, which utilizes the electrooptic effects of the TN (Twisted Nematic) type liquid crystal material sandwiched by two substrates, display images by the application of an electric field approximately vertical to the main surfaces of the substrates across the liquid crystal material to thereby cause displacement of the liquid crystal molecules of the said material. These LCD devices are termed the “vertical electric field” type. On the other hand, some of the active-matrix addressing LCD devices display images by the application of an electric field approximately parallel to the main surfaces of the substrates across the liquid crystal material to thereby cause displacement of the liquid crystal molecules of the said material in the planes parallel to the said main surfaces. These LCD devices have been known also, which are termed the “lateral electric field” type. Various improvements have ever been made not only for the vertical electric field type LCD devices but also for the lateral electric field type ones. Some of the improvements made for the latter will be exemplified below.
For example, the Patent Document 1 (Japanese Unexamined Patent Publication No. 2000-089240) published on Mar. 31, 2000 and the Patent Document 2 Japanese Unexamined Patent Publication No. 2004-062145) published on Feb. 26, 2004 disclose lateral electric field type LCD devices, each of which comprises drain bus lines and gate bus lines covered with a common electrode or electrodes in such a way that an interlayer insulating film intervenes between the drain and gate bus lines and the common electrode(s). The structure of the LCD device disclosed in the Patent Document 2 is shown in FIGS. 1, 2A to 2C, and 3.
FIG. 1 is a plan view showing the structure of the active-matrix substrate (i.e., the TFT substrate) of the said LCD device, FIGS. 2A, 2B and 2C are plan views showing the structures of the three layers that constitute the said active-matrix substrate, respectively, and FIG. 3 is an enlarged partial plan view showing the detailed structure of the vicinity of the gate bus line of the said active-matrix substrate. Since all the pixels of the active-matrix addressing LCD device have the same structure, the structure of one pixel is shown in FIGS. 1 to 3.
As clearly shown in FIGS. 2A, 2B and 2C, the active-matrix substrate of the related-art LCD device shown in FIG. 1 comprises gate bus lines 155 and common bus lines 152 formed in the same layer on a transparent insulative plate (e.g., a glass plate)(not shown); drain bus lines 156, pixel electrodes 171, TFTs 145, and storage capacitor electrodes 173 formed in the same layer on a gate insulating film (not shown) that covers the gate and common bus lines 155 and 152; and a common electrode 172 formed on a protective insulating film (not shown) that covers the drain bus lines 156, the pixel electrodes 171, the TFTs 145, and the storage capacitor electrodes 173. It is usual that the pixel electrodes 171 and the common electrode 172 are respectively formed by patterning transparent conductive metal films made of, for example, Indium Tin Oxide (ITO).
The gate bus lines 155 extending in parallel to each other at equal intervals along the lateral (horizontal) direction of FIG. 1 and the drain bus lines 156 extending in parallel to each other at equal intervals along the longitudinal (vertical) direction of the same figure define rectangular regions. Each of these rectangular regions forms a pixel region. These pixel regions (i.e., the pixels) are arranged in a matrix array as a whole. Each of the TFTs 145 is located near one of the intersections formed by the two gate bus lines 155 and the two drain bus lines 156 that define each pixel region (i.e., at the lower left intersection in FIG. 1). Similar to the gate bus lines 155, the common bus lines 152 extend along the lateral direction of the same figure in parallel with the gate bus lines 155. Each of the common bus lines 152 is located at the opposite side to the TFT 145 (i.e., at the upper end in FIG. 1) in the pixel region. In other words, it is placed near one of the two gate bus lines 155 that is located on the distant side from the TFT 145 in the pixel region, (i.e., the gate bus line 155 at the upper position in FIG. 1). Therefore, it may be said that each of the common bus lines 152 is located near the TFTs 145 existing in the preceding pixel regions that are upwardly adjacent thereto along the extension direction of the drain bus lines 156 (i.e., the vertical direction) to be apart from the said TFTs 145.
The drain electrode 144, the source electrode 142, and the semiconductor film 143 of the TFT 145 are respectively formed to have such patterns or shapes as shown in FIG. 2B. The gate electrode (not shown) of the TFT 145 is formed to be united with the gate bus line 155, in other words, the gate electrode is a part of the gate bus line 155. The gate electrode is placed at a position overlapping with the semiconductor film 143 between the drain electrode 144 and the source electrode 142. It is usual that an amorphous silicon film is used as the semiconductor film 143.
The pixel electrode 171 and the common electrode 172, which are provided for generating liquid crystal driving electric field, are formed to have such patterns or shapes as shown in FIGS. 2B and 2C, respectively. Each pixel electrode 171 and the common electrode 172 comprise comb-tooth like parts (i.e., thin belt-shaped parts protruding into the pixel region) 171a and 172a that are mated with each other, respectively. Here, the total number of the comb-tooth like parts 171a of the pixel electrode 171 is three; on the other hand, the total number of the comb-tooth like parts 172a of the common electrode 172 in each pixel region is two. The common electrode 172 further comprises openings or windows 172b formed respectively at the positions overlapped with the channel regions of the TFTs 145. For this reason, the whole channel region of the TFT 145 is exposed from the opening 172b in such a way as not to overlap with the common electrode 172. This is to avoid the change of the characteristics of the TFT 145 caused by the back gate effect.
The base of the pixel electrode 171, which is located on the side of the source electrode 142, is connected mechanically and electrically to the source electrode 142 of the TFT 145. Moreover, the ends of the three comb-tooth like parts 171a of the pixel electrode 171, which are located on the opposite side to the source electrode 142 in the pixel region, are connected mechanically and electrically to the storage capacitor electrode 173. The common electrode 172, which is commonly used for all the pixel regions, is connected electrically to the underlying common bus lines 152 by way of the corresponding contact holes 162 penetrating through the gate insulating film and the protective insulating film in the respective pixel regions.
The storage capacitor electrode 173 is placed at a position overlapped with the common bus line 152 that is directly under the electrode 173 in each pixel region, where the gate insulating film intervenes between the storage capacitor electrode 173 and the common bus line 152. The storage capacitor is formed by the overlapped parts of the storage capacitor electrode 173 and the corresponding common bus line 152. In other words, the storage capacitor is constituted by the storage capacitor electrode 173, the corresponding common line 152, and the gate insulting film intervening between them. As shown in FIG. 3, the storage capacitor is not overlapped with the gate bus line 155 that is adjacent to the corresponding common bus line 152.
As clearly seen from FIGS. 2B, 2C and 3, the common electrode 172 covers the entirety of the drain bus lines 156 extending along the vertical direction of the same figures and the entirety of the gate bus lines 155 extending along the lateral direction of the same figures (except for the openings 172b). Moreover, the common electrode 172 is formed to cover not only the areas directly above the gate bus lines 155 but also the gaps between the gate bus lines 155 and the common bus lines 152 adjacent thereto (each of the adjacent common bus lines 152 is located in the subsequent pixel regions that are downwardly adjacent thereto along the extension direction of the drain bus lines 156, i.e., the vertical direction), the gaps between the gate bus lines 155 and the corresponding source electrodes 142, the gaps between the gate bus lines 155 and the adjacent storage capacitor electrodes 173, and the peripheral areas of the edges of the source electrodes 142 and the adjacent storage capacitor electrodes 173. For this reason, the electric field generated near the gate bus lines 155 can be shielded by the common electrode 172. As seen from FIG. 3, the edges 172c of the common electrode 172 located on the side of the storage capacitor electrodes 173 (which are respectively extended along the adjacent gate bus lines 155) are not overlapped with the gate bus lines 155.
The reference numeral 181 shown in FIG. 3 denotes the black matrix layer formed on the opposite substrate. The black matrix layer 181 comprises rectangular light-shielding regions provided for the respective pixel regions. Each of the light-shielding regions is defined by a rectangular broken line in FIG. 3. Each of the light—shielding regions has a size that covers the whole TFT 145 and is isolated to have a rectangular island-like shape. In this way, the occupation area of each light-shielding region of the black matrix layer 181 is restricted to a minimum necessary for preventing the entry of light into the TFT 145. The prevention of the entry of light into (the channel region of) the TFT 145 by the light-shielding region is to prevent the functions of the TFT 145 from being hindered due to the incident light.
With the active matrix substrate of the related-art LCD device shown in FIGS. 1 to 3, as explained above, the electric field generated in the vicinities of the gate bus lines 155 can be shielded by the common electrode 172 placed in an upper layer than the gate bus lines 155. Therefore, the alignment direction of the liquid crystal molecules existing in the peripheral areas of the gate bus lines 155 is not changed from their initial alignment direction, which means that optical leakage does not occur in the same peripheral areas. Accordingly, it is unnecessary to shield the light in the same peripheral areas on the opposite substrate, and the size of each light-shielding region can be restricted to a minimum, as shown in FIG. 3.
On the other hand, with the active matrix substrate of the related-art LCD device shown in FIGS. 1 to 3, the pixel electrodes 171 may be made of the same transparent conductive metal as the common electrode 172. The structure of the active matrix substrate in this case will be explained below with reference to FIGS. 4 to 10.
FIG. 4 is a plan view showing the structure of the active-matrix substrate of the LCD device having the structure that the pixel electrodes 171 are made of the same transparent conductive metal as the common electrode 172. FIGS. 5A, 5B and 5C are plan views showing the structures of the three layers that constitute the said active-matrix substrates respectively. FIG. 6 is an enlarged partial plan view showing the detailed structure of the vicinity of the gate bus line of the said active-matrix substrate. FIG. 7 is a partial cross-sectional view of the said LCD device along the line VII-VII in FIG. 6. FIGS. 8A and 8B are partial cross-sectional views of the said LCD device along the lines VIIIA-VIIIA and VIIIB-VIIIB in FIG. 6, respectively. FIG. 9 is a plan view obtained by omitting the pixel electrode 171 and the common electrode 172 in FIG. 4 for facilitating the understanding of the understructures of them. FIG. 10 is an enlarged partial plan view obtained by omitting the pixel electrode 171, the common electrode 172, the black matrix layer 181, and the contact holes 161 and 162 in FIG. 6 for facilitating the understanding of the understructures of them. These figures also show the structure of one pixel.
As seen from FIGS. 5A, 5B and 5C, the structure of FIGS. 4 to 10 has the following differences from that of FIGS. 1 to 3: (a) The pixel electrodes 171 are made of the same transparent conductive metal as the common electrode 172 and are placed in the same layer as the common electrode 172.
(b) Auxiliary pixel electrodes 170 are formed on the same layer as the drain bus lines 156.
(c) The pixel electrodes 171 are connected electrically to the corresponding storage capacitor electrodes 173 placed in the lower layer than the pixel electrodes 171 by way of the corresponding contact holes 161 penetrating through the protective insulating film 159 (see FIG. 7 and FIGS. 8A and 8B), and furthermore, the pixel electrodes 171 are connected electrically to the corresponding source electrodes 142 by way of the corresponding auxiliary pixel electrodes 170.
The active-matrix substrate structure shown in FIGS. 4 to 10 is the same as that of FIGS. 1 to 3 except for the above-described differences (a) to (c). Therefore, explanation about the same structural elements as those of the active-matrix substrate structure of the related-art LCD device explained with reference to FIGS. 1 to 3 is omitted here by attaching the same reference numerals as used in FIGS. 1 to 3 to the same structural elements.
The pixel electrode 171 and the common electrode 172 are respectively formed to have such patterns or shapes as shown in FIG. 5C. Each pixel electrode 171 and the common electrode 172 comprise respectively comb-tooth like parts (i.e., thin belt-shaped parts protruding into the pixel region) 171a and 172a that are mated with each other in the state shown in the same figure. Here, the total number of the comb-tooth like parts 171a of the pixel electrode 171 is three; on the other hand, the total number of the comb-tooth like parts 172a of the common electrode 172 in each pixel region is two.
The auxiliary pixel electrodes 170 are formed in the same layer as the drain bus lines 156. The shape of each auxiliary pixel electrode 170 corresponds to the shape formed by the combination of the base of the pixel electrode 171 in the structure shown in FIGS. 1 to 3 and the central comb-tooth like part 171a thereof. The bottom end of the auxiliary pixel electrodes 170 is connected mechanically and electrically to the source electrode 142 and the top end thereof is connected mechanically and electrically to the storage capacitor electrode 173. In this way, the pixel electrode 171 is electrically connected to the source electrode 142 by way of the storage capacitor electrode 173 and the auxiliary pixel electrode 170 in the pixel region.
As shown in FIGS. 9 and 10, the storage capacitor electrode 173 is overlapped with the corresponding common bus line 152 located right below the said storage capacitor electrode 173; however, the storage capacitor electrode 173 is not overlapped with the adjacent gate bus line 155. The common electrode 172 covers the entirety of the corresponding gate bus line 155 and therefore, the edge 172c of the common electrode 172 (which is extended along the adjacent gate bus line 155) located on the side of the storage capacitor electrode 173 is not overlapped with the adjacent gate bus line 155. This point is the same as the structure of FIGS. 1 to 3.
Next, the entire configuration of the related-art LCD device shown in FIGS. 4 to 10 will be explained below with reference to FIG. 7 and FIGS. 8A and 8B.
This LCD device is configured by coupling and uniting the active-matrix substrate and the opposite substrate with each other in such a way that a liquid crystal layer 120 intervenes between these two substrates.
The active-matrix substrate comprises a transparent glass plate 111; and the common bus lines 152, the gate bus lines 155, the drain bus lines 156, the TFTs 145, the auxiliary pixel electrodes 170, the pixel electrodes 171, the common electrode 172, and the storage capacitor electrodes 173 formed on or over the inner surface of the glass plate 111. The common bus lines 152 and the gate bus lines 155, which are placed directly on the inner surface of the glass plate 111, are covered with the gate insulating film 157 except for the positions corresponding to the contact holes 162. The drain electrodes 144, the source electrodes 142, and the semiconductor films 143 of the TFTs 145; the auxiliary pixel electrodes 170; the storage capacitor electrodes 173; and the drain bus lines 156 are placed on the gate insulating film 157. Therefore, the common bus lines 152 and the gate bus lines 155 are electrically insulated from the drain electrodes 144, the source electrodes 142, and the semiconductor films 143, the auxiliary pixel electrodes 170, the storage capacitor electrodes 173, and the drain bus lines 156 by the gate insulating film 157. These structures formed on the glass plate 111 are covered with the protective insulating film 159 except for the positions corresponding to the contact holes 161 and 162.
The pixel electrodes 171 and the common electrode 172 are placed on the protective insulating film 159. As explained above, the pixel electrode 171 is electrically connected to the corresponding storage capacitor electrode 173 located right under the same pixel electrode 171 by way of the corresponding contact hole 161 (which penetrates through the protective insulating film 159) and to the corresponding source electrode 142 by way of the corresponding auxiliary pixel electrode 170 in the pixel region. The common electrode 172 is electrically connected to the common bus lines 152 located right under the common electrode 172 by way of the corresponding contact holes 162 (which penetrates through the protective insulating film 159 and the gate insulating film 157) in the respective pixel regions. The pixel electrodes 171 and the common electrode 172 are respectively formed by pattering transparent conductive metal films, for example, ITO films.
The surface of the active-matrix substrate having the above-described structure (i.e., the surface on which the pixel electrodes 171 and the common electrode 172 are formed) is covered with an alignment film 131 made of an organic polymer. The surface of the alignment film 131 has been subjected to a predetermined aligning treatment for aligning the initial alignment direction of the liquid crystal molecules existing in the liquid crystal layer 120 to a desired direction.
On the other hand, the opposite substrate (which may be termed the color filter substrate) comprises a transparent glass plate 112; a color filter (not shown) formed by three color layers 182R, 182G, and 182B of the three primary colors, i.e., red (R), green (G) and blue (B), formed on the inner surface of the glass plate 112 corresponding to the arrangement of the respective pixel regions; and the black matrix layer 181 for optical shielding. Similar to the structure of FIGS. 1 to 3, the black matrix layer 181 comprises the rectangular light-shielding regions for the respective pixel regions, each of which is defined by the broken line in FIG. 6. In addition, the three color layers 182R, 182G, and 182B are generically termed the color layer 182.
The color layer (i.e., the color filter) 182 and the black matrix layer 181 are covered with an overcoat layer 185 made of an acrylic resin. Columnar spacers (not shown) are formed on the inner surface of the overcoat layer 185 to keep the gap between the active-matrix substrate and the opposite substrate. The inner surface of the overcoat layer 185 is covered with an alignment film 132 made of an organic polymer. The surface of the alignment film 132 has been subjected to a predetermined aligning treatment for aligning the initial alignment direction of the liquid crystal molecules existing in the liquid crystal layer 120 to a desired direction.
The active-matrix substrate and the opposite substrate each having the above-described structure are superposed on each other at a predetermined gap in such a way that their surfaces on which the alignment films 131 and 132 are respectively formed are directed inwardly and opposed to each other. The liquid crystal layer 120 is formed in the gap between the active-matrix and opposite substrates. To confine the liquid crystal material existing in the liquid crystal layer 120 into the gap between the two substrates, the outer edges of the two substrates are sealed with a sealing material (not shown). A pair of polarizer plates (not shown) is arranged on the outer surfaces of the two substrates, respectively.
In addition, the Patent Document 3 (Japanese Unexamined Patent Publication No. 2000-029014) published on Jan. 21, 2000 and the Patent Document 4 (Japanese Unexamined Patent Publication No. 2002-082630) published on Mar. 22, 2002 disclose the technique for forming the light-shielding regions of the black matrix layer 181 by overlapping the end portions of the adjoining color layers of the color filter, where the black matrix layer 181 is not used. If this technique is employed, the formation processes of the black matrix layer 181 can be omitted and as a result, the fabrication cost lowering is realizable.
With the above-described two structures of the related-art LCD devices, the whole surfaces of the respective gate bus lines 155 are covered with the common electrode 172 placed in the upper layer than the gate bus lines 155. This is to prevent the optical leakage caused by the alignment direction change of the liquid crystal molecules from their initial alignment direction in the peripheral areas of the gate bus lines 155 and the TFTs 145 due to the electric field generated in the same peripheral areas. However, in the case where the whole surfaces of the respective gate bus lines 155 are covered with the common electrode 172 in this way, there is a problem that the degree of freedom in designing the pattern and layout of the respective constituent elements of these two related-art LCD devices is low and as a result, it is difficult to improve the aperture ratio.
As another method of preventing such the optical leakage as explained above, a method of broadening the respective light-shielding regions of the black matrix layer 181 that are arranged on the opposite substrate at the predetermined positions overlapped with the respective gate bus lines 155 is known. In this method, however, it is necessary to broaden the respective light-shielding regions sufficiently in consideration of the margins for the positional deviation occurring in the coupling operation of the active matrix substrate and the opposite (or color filter) substrate. Accordingly, in this case also, it is difficult to realize a high aperture ratio.
In the case where the light-shielding regions are formed by overlapping the end portions of the different color layers of the color filter instead of the use of the black matrix layer 181, as disclosed in the Patent Documents 3 and 4, fabrication cost lowering may be realized due to the omission of the formation processes of the black matrix layer 181. In this case also, however, it is necessary to form the sufficiently wide light-shielding regions on the opposite substrate in consideration of the margins for the positional deviation between the active-matrix substrate and the opposite substrate. Therefore, it is difficult to realize a high aperture ratio.
Furthermore, there is another problem that the large level difference formed by overlapping the end portions of the different color layers affects badly the alignment of the liquid crystal molecules and/or prolong the time required for the injection process of the liquid crystal material.